[flang][FIR] allow mem2reg over fir.declare (#181848)
This patch adds the possibility for MLIR mem2reg to work over
fir.declare.
Note that mem2reg is not part of FIR pipeline, and this is just part of
work to be able to leverage it.
The patch:
- Adds a fir.declare_value operation
- Implements the PromotableOpInterface for fir.declare simple scalars
and replace it by fir.declare_value.
- Generates llvm.dbg.debug_value from it (when a FusedLoc with a
DILocalVariableAttr is created for it in AddDebugInfo, like for
fir.declare).
